Add reading of externally re-gridded gsi land increment files

[ClaraDraper-NOAA]

In global_cycle, the current code can read in land increments either:
i) created by jedi on the native model grid, or
ii) created by gsi on the Gaussin grid, and then regridded within global_cycle.

The gridding routines in global_cycle cannot account for land/sea masks.To take advantage of external regridding code that handles masked fields, global_cycle needs to be updated to also handle gsi-generated land increment files that have already been re-gridded.